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NCAR Tip Sheet: Bumper Crop Of Atlantic Hurricanes?

Date:

August 9, 1999

Source:

National Center For Atmospheric Research (NCAR)

Summary:

This year may bring a bumper crop of Atlantic hurricanes, thanks to the
influence of La Nina and other factors. This release includes a range
of Web sites, answers to frequently asked questions, and a set of
hurricane experts from the National Center for Atmospheric Research
(NCAR) and other institutions.

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BOULDER -- This year may bring a bumper crop of Atlantic hurricanes,thanks to the influence of La Nina and other factors. Below is a rangeof Web sites, answers to frequently asked questions, and a set ofhurricane experts from the National Center for Atmospheric Research(NCAR) and from member institutions of the University Corporation forAtmospheric Research (UCAR). NCAR's primary sponsor is the NationalScience Foundation. NCAR is managed by UCAR, a consortium of more than60 universities offering Ph.D.s in atmospheric and related sciences.

Specialty: Hurricane impacts and related societal factors. A politicalscientist, Pielke has completed in-depth studies of the societalvulnerability to hurricanes of the U.S. Gulf and Atlantic coasts. In theOctober 1999 issue of the Bulletin of the American Meteorological Society,Pielke and colleague Christopher Landsea (see listing below) will outlinethe relationship of El Nino and La Nina to U.S. hurricane damage. They notethat most La Nina years (17 out of 22)--but only a few El Nino years (7 outof 22)--produce a U.S. landfall causing at least $1 billion in damage.Catastrophic storms of $10 billion or more are so rare that theirrelationships to El Nino or La Nina are difficult to assess. The study ison line at http://www.dir.ucar.edu/esig/HP_roger/bams. Pielke and Landseahave also examined trends in population and wealth to calculate the costsof historic U.S. hurricanes in 1997 dollars. Pielke is the co-author ofHurricanes: Their Nature and Impacts on Society (Wiley, 1997) and creatorof the Extreme Weather Sourcebook (see Web sites below).

Specialty: Analysis of tropical systems. Through radar analysis, Gall hasuncovered fine-scale bands that spiral around the centers of hurricanes.These bands could be related to small-scale wind peaks that cause some of ahurricane's worst damage. Gall and fellow NCAR scientist John Tuttle havealso developed a technique that uses radar data to estimate surface windsat points that lie beyond the typical range of Doppler radar. Gall is nowserving a two-year term as lead scientist of the U.S. Weather ResearchProgram, a multiagency project to examine high-impact phenomena, includinghurricane landfall.

Specialty: Measurement of winds through Global Positioning System (GPS)dropsondes. Hock led NCAR's development of the revolutionary GPS device.The dropsondes have been deployed over the past three years from hurricane-hunting aircraft. They report winds between flight level and the seasurface at 16-foot (5-meter) intervals, a precision about 100 times greaterthan before. The new dropsondes have gathered the first-ever wind data athigh resolution from the eyewall that swirls around the calm eye of ahurricane.

Specialty: Seasonal hurricane forecasting and hurricane climatology.Landsea is a collaborator with hurricane forecaster William Gray (ColoradoState University). He has studied patterns of hurricane occurrence anddamage along the Gulf and Atlantic coasts over the past century and is theauthor of a frequently-asked-questions document on the World Wide Web (seebelow).

Specialty: Human response to hurricanes, including evacuations. Through on-site visits and interviews after hurricanes, Baker has examined how peoplerespond to warnings and evacuation orders. He also has studied howemergency managers use forecasts and other information to implementevacuation plans.

This site includes official outlooks, climatology, and statistics onhurricanes, tropical storms, and tropical depressions. TPC includes theNational Hurricane Center, which issues hurricane watches and warnings andcalculates official projections of storm tracks.

This site offers yearly totals and state-by-state comparisons for hurricanedamages between 1925 and 1995. The figures are normalized to account fortrends in population and wealth. Florida, Texas, and Louisiana are theleaders for hurricane damage, with Florida averaging over $2 billion peryear.

This site, from Colorado State University, includes-- the latest long-range hurricane outlooks from CSU professor William Gray-- a comprehensive set of answers to frequently asked questions about hurricanes and other tropical cyclones, compiled by Chris Landsea.

This site includes a wealth of links to Web-based hurricane information.Because popular Web sites often become busy and difficult to access when amajor hurricane approaches, the author includes several alternate addressesfor frequently sought items, such as the latest storm intensities andprojected tracks.

Background information on hurricanes

What is a hurricane? Each year a number of tropical disturbances--centers of low pressure--move westward across the Atlantic, Pacific, andIndian Oceans. Some of these become depressions--more organizeddisturbances with sustained surface winds of up to 38 miles per hour (61kilometers per hour). Should the winds become stronger, the system becomesa tropical storm and is given a name. If the winds reach 74 mph (119km/hr), the storm is reclassified as a hurricane (other names, such astyphoon, are used outside of the Atlantic). Eventually, most of thesesystems either strike land and quickly weaken or recurve over the ocean,moving north and east as they become caught up in the midlatitude westerlywinds and lose their tropical characteristics.

What's the difference between a hurricane and a tornado?Tornadoes are spawned by thunderstorms, while hurricanes are made up ofmany showers and thunderstorms (which can themselves spawn tornadoes uponlandfall). Hurricanes gather energy from the warmth of the ocean. Ahurricane's eye is typically 10 to 15 miles (16 to 24 km) wide, and windsaround it are as strong as 150 to 200 mph (240 to 320 km/hr). The hurricanecirculation can be hundreds of miles across. Even the largest tornadoes areonly about a mile across, although their winds can reach 300 mph (480km/hr). Tropical storms can survive for weeks, while most tornadoes existfor much less than an hour.

How many tropical systems occur on average each year? Globally, anaverage of around 85 tropical storms and 45 hurricanes/typhoons form peryear. The tropical Atlantic and Gulf of Mexico--the sources of U.S.hurricanes--produce an average of nine named storms per year, with aroundsix of those becoming hurricanes and two of those becoming intensehurricanes (ones with sustained surface winds exceeding 110 mph, or 177km/hr). The period since 1995 has been unusually active. Even though 1997saw few Atlantic storms, the 1995-1998 total was the largest on record fora four-year span in the Atlantic, with 53 named storms, including 32hurricanes.

How are hurricanes predicted? Some climate factors are known to affecthurricane frequency for a given ocean or a given year. For instance, ElNino enhances upper-level winds that tend to suppress Atlantic hurricanes,while La Nina has the opposite effect. Each year, out of dozens ofdisturbances that cross the tropical Atlantic, only a handful encounter theright combination of light wind shear and warm ocean temperatures thatallows for hurricane development. The behavior of tropical storms andhurricanes is predicted by high-resolution computer models at the NOAATropical Prediction Center, the federal agency that issues hurricanewatches and warnings (see Web site above). Hurricane motion can beprojected with some skill out to five days. Changes in storm intensity arestill difficult to predict, even within a day or two, but some progress hasbeen made with better computer models. New observing tools, such as GPSdropsondes (see Terry Hock, above) and satellite-based radars that cansense unusually deep layers of warm water, are also helping to improveforecasts by providing a better picture of current conditions.

What are the storm names for this year and upcoming years? Since1953, Atlantic tropical storms have been named from lists originated by theNational Hurricane Center and now maintained and updated by Region 4 of theWorld Meteorological Organization. The lists featured only women's namesuntil 1979. Six lists are used in rotation. The names of the most severehurricanes, such as 1998's Mitch, are retired permanently. Below are thelists for 1999-2004.

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